Neuropeptide Y is an inhibitor of neural function in the myenteric plexus of the guinea-pig ileum.

Neuropeptide Y (NPY) reduced the resting tension of the myenteric plexus-longitudinal muscle preparation (MP-LM) of the guinea-pig ileum (GPI). NPY in a dose-dependent manner also reduced the neurally-mediated excitatory effect of cholecystokinin octapeptide (CCK8) sulfated form on this preparation. However, NPY, at the concentration used in the study, did not modify the effect of exogenous acetylcholine (ACh). All these features were also shared by other inhibitory peptides, like somatostatin (SOM) and the enkephalin derivative FK 33-824. The preparation developed a degree of tachyphylaxis to the inhibitory effect of NPY more rapidly than it did to SOM. Moreover, the inhibitory effect of neuropeptide Y was of longer duration than the one seen for somatostatin. A faster metabolic rate might account for the lower development of tachyphylaxis to somatostatin. The presence of the opioid antagonist naloxone did not alter the inhibitory features of NPY or SOM. Therefore, the involvement of any endogenous opioid in the action of these two inhibitory peptides can be disregarded.